The present invention relates to methods for processing sonic waveform measurements, particularly sonic waveform measurements made for the purpose of characterising properties of underground formations. The invention in particular relates to methods for processing sonic measurements made with array tools.
Sonic logging tools for use in characterising underground formations surrounding a borehole by measuring the effect of the formation on a sonic signal propagating through it are well known. Such tools commonly include an array of transducers, commonly receivers in order to improve the sensitivity and accuracy of the measurements made. One example of an array tool for use in a borehole for sonic measurements is the DSI tool of Schlumberger which is shown schematically in FIG. 1. The DSI tool comprises a transmitter section 10 having a pair of (upper and lower) dipole sources 12 arranged orthogonally in the radial plane and a monopole source 14. A sonic isolation joint 16 connects the transmitter section 10 to a receiver section 18 which contains an array of eight spaced receiver stations, each containing two hydrophone pairs, one oriented in line with one of the dipole sources, the other with the orthogonal source. An electronics cartridge 20 is connected at the top of the receiver section 18 and allows communication between the tool and a control unit 22 located at the surface via an electric cable 24. With such a tool it is possible to make both monopole and dipole measurements. The DSI tool has several data acquisition operating modes, any of which may be combined to acquire (digitised) waveforms. The modes are: upper and lower dipole modes (UDP, LDP)xe2x80x94waveforms recorded from receiver pairs aligned with the respective dipole source used to generate the signal; crossed dipole modexe2x80x94waveforms recorded from each receiver pair for firings of the in-line and crossed dipole source; Stoneley modexe2x80x94monopole waveforms from low frequency firing of the monopole source; P and S mode (PandS)xe2x80x94monopole waveforms from high frequency firing of the monpole transmitter; and first motion modexe2x80x94monopole threshold crossing data from high frequency firing of the monopole source.
One way to determine compressional, shear and Stoneley slownesses from these measurements is to use slowness-time-coherence (STC) processing. STC processing is a full waveform analysis technique which aims to find all propagating waves in the composite waveform. The processing adopts a semblance algorithm to detect arrivals that are coherent across the array of receivers and estimates their slowness. The basic algorithm advances a fixed-length time window across the waveforms in small, overlapping steps through a range of potential arrival times. For each time position, the window position is moved out linearly in time, across the array of receiver waveforms, beginning with a moveout corresponding to the fastest wave expected and stepping to the slowest wave expected. For each moveout, a coherence function is computed to measure the similarity of the waves within the window. When the window time and the moveout correspond to the arrival time and slowness of a particular component, the waveforms within the window are almost identical, yielding a high value of coherence. In this way, the set of waveforms from the array is examined over a range of possible arrival times and slownesses for wave components. STC processing produces coherence (semblance) contour plots in the slowness/arrival time plane. Regions of large coherence correspond to particular arrivals in the waveforms. The slowness and arrival time at each coherence peak are compared with the propagation characteristics expected of the arrivals being sought and the ones that best agree with these characteristics are retained. Classifying the arrivals in this manner produces a continuous log of slowness versus depth. For dispersive waves, the STC processing is modified to take into account the effect of frequency. As the output of STC processing is a coherence plot, the coherence of each arrival can be used as a quality indicator, higher values implying greater measurement repeatability. When processing dipole waveforms, one of the coherence peak will correspond to the flexural mode but with a slowness that is always greater (slower) than the true shear slowness. A precomputed correction is used to remove this bias. Further details of STC processing can be found in Kimball C. V. and Marzetta T. L., xe2x80x9cSemblance processing of borehole acoustic array dataxe2x80x9d Geophysics, Vol. 49, No. 3 (March 1984); pp 274-281.
To compensate for variations in measurements due to the borehole rather than due to the formation a series of measurements are made across an interval in which the formation properties are expected to vary little, if at all. In its simplest form, the interval corresponds to the extent of the receiver array, and the waveforms at each receiver station measured for a given firing of a source (xe2x80x9creceiver arrayxe2x80x9d or xe2x80x9creceiver modexe2x80x9d or xe2x80x9cRec.xe2x80x9d). In simple STC processing, all receiver stations are considered. In multishot STC processing (MSTC), sub-arrays of receiver stations within the receiver array are considered, for example a sub-array of five receiver stations in a receiver array of eight receiver stations (other numbers or receiver stations in the sub-array can be used depending on requirements). In this case, the same formation interval corresponding to the extent of a five receiver station sub-array can be measured several times as the tool is logged through the borehole, the five stations making up the sub-array being selected at each source firing to measure the same formation interval. Another approach, known as xe2x80x9ctransmitter modexe2x80x9d or xe2x80x9cpseudo-transmitter arrayxe2x80x9d (xe2x80x9cTra.xe2x80x9d) takes waveforms from sequential source firings as the transmitter passes along the interval to be measured. In order to compensate for the movement of the tool between measurements, an effectively stationary receiver station or sub-array must be used. This can be achieved by changing the receiver station considered so that its position in the borehole is effectively stationary as the transmitter is moved through the interval. Borehole compensation (xe2x80x9cBHCxe2x80x9d) can be achieved for P and S mode results by processing receiver array and pseudo-transmitter array waveforms and averaging the results. A schematic example of an eight receiver array tool making measurements in across a depth interval five receivers in length for successive firings of a transmitter is shown in FIG. 2. By using the five receiver sub-array, four measurements are made of the same depth interval with the transmitter in positions i, i+1, i+2 and i+3. STC processing of each sub-array produces a series of slowness time plots as shown in FIG. 3(a). If it is desired to combine the information obtained from this series of transmitter firings, it is necessary to deal with the time shift between each sub-array due to the movement of the source. Previously this has been achieved by projecting each slowness time plot onto the slowness axis (FIG. 3(b)) followed by stacking the four plots into one (FIG. 3(c)). Further details of this approach can be found in Hsu, K. and Chang, S. K., xe2x80x9cMultiple-shot processing of array sonic waveformsxe2x80x9d Geophysics, Vol. 52, No. 10 (October 1987); pp 1376-1390. It will be appreciated that collapsing the separate ST plots onto the slowness axis gives up useful information derived from the time axis of each plot. U.S. Pat. No. 4,543,648 also describes methods of shot to shot processing of sonic waveforms.
This invention provides methods for processing sonic logging data obtained from a logging tool having an array of R receivers, comprising:
selecting a sub-array size of Rsb receivers where Rsb less than R;
calculating the number of sub-arrays Nsb possible from the array of R receivers;
identifying one receiver Rref in the array to serve as a reference receiver for each sub-array;
processing data in the slowness-time plane for each sub-array with respect to the reference receiver Rref, and
stacking the processed data in the slowness-time plane for all sub-arrays.
Stacking can be achieved using algebraic or geometric summation, an algebraic mean being preferred.
Processing in the slowness time plane can comprise stacking slowness-time coherences for each sub-array, or processing data to obtain stacks comprising the coherent energy and total energy in the slowness time plane for the sub-arrays and then obtaining the ratio of the two to derive the stacked slowness-time coherence data.
It is preferred to use the maximum number of sub-arrays possible for a given array receiver in order to maximise the amount of information obtained from the formation. For an array of R (sometimes given below as Nrcvr)receivers, the maximum number of sub-arrays Nsb, of Rsb receivers in length, is given by
Nsb=Rxe2x88x92Rsb+1.xe2x80x83xe2x80x83(1)
For an eight receiver array tool, a five receiver sub-array size allows four sets of waveforms to be recorded for a given depth.
Identification of the traces (waveforms) to be recorded can be achieved by computing a geometric matrix G of dimensions Nsb by Rsb, which gives the receivers to be considered at each shot at a given depth. Identification of which traces from the whole array for each shot can be achieved by computing a multiple-shot matrix M of dimensions
Nsb, Nrcvr+Nsbxe2x88x921.xe2x80x83xe2x80x83(2)
Traces not considered from a given shot are assigned a 0 values in such a matrix.
The reference receiver rref for a set of sub-arrays is identified from the relationship (1), where Nsb also identifies the position of the reference receiver rref in the array.